The difference in phase between the voltage supplied to an induction motor and the resulting current through the motor, known as the power factor, is indicative of the load on the motor. It is known for a power control system to be connected to a motor in order to detect and compare the supplied-voltage and resulting-current signals. Based upon this comparison, the power control system may control the voltage applied to the motor, which in turn controls the flow of current to the motor, in order to reduce the power consumed by a less than fully loaded motor.
U.S. Pat. No. 4,266,177 to Nola, for example, describes a power control circuit for an induction motor (hereinafter “the Nola '177 circuit”), wherein a servo loop is used to control the voltage applied to the motor, which in turn controls the flow of current to the motor, in order to reduce the power consumed by the motor. In particular, a pulse signal is used to control the “on” time of a triac which is in circuit with the motor in order to maintain motor operation at a selected power factor. The pulse signal is based upon the measured current-voltage phase angle.
Power factor controllers of the prior art, such as the one just described, use an integrator as part of the processing required to produce the pulse signal. Typically, the integrator includes an operational amplifier and a filter which includes a capacitor and provides a single path of feedback from the output of the operational amplifier to one of the inputs of the operational amplifier. A command signal circuit is also connected to one of the inputs of the operational amplifier, which is typically the same input to which the filter is connected. Conventionally, the command signal circuit contains a potentiometer. The potentiometer must be adjusted for the particular motor being controlled in order to provide a proper bias voltage to the operational amplifier. In effect, it sets a selected power factor (or phase angle between current and voltage) as determined by the greatest power factor (smallest motor current-voltage phase difference) at which the motor will operate over a range of loadings to be encountered. The resulting control signal is a negative signal which shifts positively responsive to the presence of a higher than commanded power factor, and shifts negatively when there is detected a lower than commanded power factor. It is employed in a servo loop to vary the applied voltage and control the input power to the motor. In this way, the motor is forced to operate at the selected power factor. In such circumstances, this enables motors which are less than fully loaded to draw significantly reduced power.
Power factor controllers require a power supply in order to provide an operating bias voltage of, for example, 15 volts, to the controller's active components, such as the operational amplifier of the integrator, so that the pulse signal is provided to the triac Exemplary transformer-less power supplies are employed. However, in such cases, relatively large capacitors are typically necessary, and this slows full voltage output and start-up time of the circuitry. This in turn may prevent a motor from having a sufficient starting voltage (average voltage through triac) initially applied to it for effective starting. To compensate for this, a delay circuit is employed which delays any power from being applied to motor until operating biases are essentially at full operating levels.
Unfortunately, the delays provided by such delay circuits may be too short during times of high temperature stresses, as would occur during summer months for air conditioning system and refrigeration systems. In such circumstances, the voltage supplied by the power company is lowered in response to the heavy loads produced by the very same air conditioning systems. The voltage supplied by the power companies is lowered to just above the level of adequate operation of such systems. Further, these air conditioning and refrigeration systems need even more time for the pressures in the compressor portion of such systems to stabilize and for back pressures to have been eliminated. It is therefore necessary to apply as much of the full supply voltage as possible for longer periods of time than can be provided by the time delay circuits employed in existing power factor controllers based on the Nola '177 circuit. Any energy saving circuit/power factor controller operating in a similar fashion to the Nola '177 circuit would lower the voltage supplied to AC induction motors even further, often leading to catastrophic failures of the AC induction motors.
Accordingly, what would be desirable, but has not yet been provided, is a system for delaying the operation of energy savings/power factor controller, such as the Nola '177 circuit, or conversely causing such circuits to apply maximum available supply voltage for a longer period of time that has been previously provided until the system employing the AC induction motor has stabilized.